The mobile phone-sized device uses GPS to automatically detect when you're entering a Congestion Charging zone in the city and automatically pays the appropriate fee by using financial details stored at the KenBuster data centre.

The KenBuster could spell financial trouble for TfL as it'll no longer earn a third of its cash from fines.

"Based on results from 2005/2006 TfL makes a total revenue of £210 million of which approximately £65 million is made up of Congestion Charge fines," the KenBuster company said in a press statement.

"Approximately 4,500 people are fined per day, equating to roughly six percent of those driving in the congestion zone, yet TfL generates over 31 per cent of revenue from fines."

Avoid Congestion Charge fines

Drivers are fined because often don't realise they've entered a Congestion Charging zone, don't realise they can pay online in advance or forget to pay the same day.

The KenBuster costs £50 to buy, plus an £8 per month subscription (12 month contract). You can also buy it outright with no additional fees for £199.